Estimating population sizes can be tricky


  • Various techniques based on biology of organism
    • enumeration method: capture all animals
    • population surveys: deer counts


  • Many organisms are not easy to count in a straightforward was
    • aquatic organisms
    • small animals
    • elusive animals


  • Often we use indirect methods to calculate population size

Depletion method (DeLury)




  • Popular for fish estimates


  • Methods relies on:
    • CPUE: Catch per unit effort
    • Cumulative catch


  • The X intercept gives an approximate measure of population size.

Assumptions of DeLury method

  • Catchability remains the same


  • Intensity of effort stays the same
    • methods of capture do not interfere


  • Population is constant
    • no recruitment or immigration
    • no births or deaths


  • All organisms are equally vulnerable to capture


  • Linear line describes the data!

Mark and Re-capture method

Lincoln-Peterson index

Assumptions of Mark and Re-Capture

  • All organisms are equally vulnerable to capture


  • Marked individuals are randomly dispersed


  • Individuals with marks have the same probability of survival


  • Marks do not come off


  • Recapture rates are high enough to support an accurate estimate


  • Population is constant
    • no recruitment or immigration
    • no births or deaths

Water Strider Biology

  • Predatory insects that skate on the surface of slow-moving water


  • To mobile to count easily


  • Not overly difficult to catch with nets
    • Delury method possible


  • Thorax large enough to mark
    • Mark Re-Capture possible

Open up the excel file!